Optical film stack including retardation layer

ABSTRACT

Optical film stacks are described. More particularly, optical film stacks including a half-wave retardation layer are described. Achromatic half-wave retardation layers, including achromatic half-wave layers formed from a quarter-wave and a three-quarters-wave retardation layer, are described. Film stacks including reflective polarizers tuned to reduce wavelength dispersion of the half-wave retardation layer are also described.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage filing under 35 U.S.C. 371 ofPCT/US2014/067165, filed Nov. 24, 2014, which claims the benefit of U.S.Provisional Application No. 61/908,396, filed Nov. 25, 2013, thedisclosure of which is incorporated by reference in its entirety herein.

BACKGROUND

In backlights, reflective polarizers are used to recycle light andenhance the ultimate efficiency and brightness of the display. Absorbingpolarizers or the like are used in conjunction with a liquid crystalmodule to polarize the light for appropriate modulation by the liquidcrystal module. In some manufacturing processes, reflective polarizersin roll form may have their reflective axis (i.e., block axis or slowaxis) in the transverse direction (i.e., the width direction of theroll). Absorbing polarizers in roll form have their absorption axisalong the length direction of the roll (i.e., machine direction). Incases where it may be desirable to align the reflection axis and theabsorption axis within a film stack, one or both of the rolls ofpolarizers will need to be cut and rotated, adding manufacturing timeand process expense.

SUMMARY

In one aspect, the present disclosure relates to an optical film stack.The optical film stack includes a reflective polarizer having a topsurface, a bottom surface, a transmission axis, and a reflection axis.The optical film stack also includes an achromatic half-wave retardationlayer having a top surface, a bottom surface disposed on the top surfaceof the reflective polarizer, and a slow axis oriented substantially 45°with respect to the transmission axis of the reflective polarizer. Theoptical film stack also includes an absorbing polarizer having a bottomsurface disposed on the top surface of the achromatic half-waveretardation layer and a transmission axis oriented substantially 90°with respect to the transmission axis of the reflective polarizer.

In another aspect, the present disclosure relates to an optical filmstack including a reflective polarizer having a top surface, a bottomsurface, a transmission axis, and a reflection axis. The optical filmstack also includes a first retardation layer having a top surface, abottom surface disposed on the top surface of the reflective polarizer,and a slow axis oriented substantially 45° with respect to thetransmission axis of the reflective polarizer, and a second retardationlayer having a top surface, a bottom surface disposed on the top surfaceof the first retardation layer, and a slow axis oriented substantially45° with respect to the transmission axis of the reflective polarizerand oriented substantially 90° with respect to the slow axis of thefirst retardation layer. The optical film stack also includes anabsorbing polarizer having a bottom surface disposed on the top surfaceof the second retardation layer and a transmission axis orientedsubstantially 90° with respect to the transmission axis of thereflective polarizer. Together the first and second retardation layersare configured as an achromatic half-wave retardation layer. In someembodiments, the first retardation layer is a three-quarters-waveretardation layer and the second retardation layer is a quarter-waveretardation layer. In some embodiments, the second retardation layerincludes a liquid crystal layer. In some embodiments, the firstretardation layer includes a film. In some embodiments the liquidcrystal layer has a thickness of between 1 and 1.3 microns, and in someembodiments the liquid crystal layer has a thickness of between 1.18 and1.24 microns.

In yet another aspect, the present disclosure relates to an optical filmstack including a tuned reflective polarizer having a top surface, abottom surface, a transmission axis, and a reflection axis, a half-waveretardation layer having a top surface, a bottom surface disposed on thetop surface of the reflective polarizer, and a slow axis orientedsubstantially 45° with respect to the transmission axis of thereflective polarizer, and an absorbing polarizer having a bottom surfacedisposed on the top surface of the achromatic half-wave retardationlayer and a transmission axis oriented substantially 90° with respect tothe transmission axis of the reflective polarizer. The tuned reflectivepolarizer is tuned to reduce wavelength dispersion of the half-waveretardation layer.

In some embodiments, the optical film stack also includes a quarter-waveretardation layer disposed on the bottom surface of the reflectivepolarizer. In some embodiments, a retardation value of the achromatichalf-wave retardation layer varies directly with a wavelength of visiblelight. In some embodiments, substantially 45° means not less than 35°and not more than 55°, not less than 40° and not more than 50°, or notless than 44° and not more than 46°. In some embodiments, substantially90° means not less than 80° and not more than 100°, not less than 85°and not more than 95°, or not less than 89° and not more than 91°. Theoptical film stack may be in roll form.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded top perspective view of an optical film stackincluding a retardation layer.

FIG. 2 is an exploded front elevation view of the optical film stack ofFIG. 1.

FIG. 3 is an exploded top perspective view of another optical film stackincluding a retardation layer.

FIG. 4 is a graph showing transmittance as a function of wavelength forvarious thicknesses of liquid crystal polymer in exemplary optical filmstacks.

FIG. 5 is a graph showing color shift at various viewing angles as afunction of liquid crystal thickness for exemplary optical film stacks.

FIG. 6 is a graph showing transmittance as a function of wavelength fordifferent retarder alignments.

FIG. 7 is a graph showing color shift as a function of retarderalignment for different viewing angles.

FIG. 8 is a graph showing transmittance as a function of wavelength fordifferent liquid crystal and cyclo-olefin polymer film alignments inexemplary optical film stacks.

DETAILED DESCRIPTION

In some backlight applications, it may be useful to utilize an opticalfilm stack that has similar performance to a standard reflectivepolarizer/absorbing polarizer construction, but is able to bemanufactured through a roll to roll process. Further, it may be usefulto utilize an optical film stack that uses standard rolls of reflectivepolarizer film and absorbing polarizer film and can be delivered andstored in roll form.

In conventional manufacturing processes and with conventional materialselections, reflective polarizers are stretched such that the reflectionaxis (that is, the axis parallel to the polarization statepreferentially reflected by the reflective polarizer) is along the widthdirection of the film. In film line parlance, this is referred to as thetransverse direction (in contrast to the machine direction, along thelength of the film).

Conventional dye-stock absorbing polarizers, in contrast toconventionally manufactured reflective polarizers, are aligned in themachine direction, resulting in its absorption axis being substantiallyalong the length of the absorbing polarizer film.

In some backlights, a liquid crystal module is used to modulate thepolarization of light. These liquid crystals may have differentorientations and degrees of twist depending on the type of liquidcrystal and whether voltage is applied across the liquid crystal module.For example, in a twisted nematic type liquid crystal module, the liquidcrystal molecules' structures is such that in an off state (that is,when no voltage is applied), the liquid crystal rotates or modulates thepolarization of the light by 90°. In the on state, that is, whensufficient voltage to orient the liquid crystal modules is applied, theliquid crystal molecules are aligned and do not rotate or module thatpolarization of incident light. In these, crossed polarizers, forexample, are disposed on either side of the liquid crystal module. Thus,light may be blocked in the on state because the light remainsunmodulated and is extinguished by the polarizer oriented at 90°.Likewise, light may be substantially transmitted in the off state as thepolarization is rotated 90° to substantially align with the with thetransmission axis of the absorbing polarizer.

In some of these constructions, a reflective polarizer and an absorbingpolarizer are disposed on one another in order to provide desirableoptical characteristics. For example, a reflective polarizer can be usedin conjunction with a highly reflective film or surface, providing alight recycling cavity. Light having a polarization state that ispreferentially transmitted through the reflective polarizer is passedwhile light having an orthogonal polarization state is preferentiallyreflected. Light having the reflected polarization state may bereflected back and forth (ideally with little absorptive loss) until ithas the preferentially transmitted polarization state. This may minimizewasted light that is simply absorbed by an absorbing polarizer, insteadensuring that the maximum useful light is transmitted. The absorbingpolarizer may provide helpful anti-reflection or defect hidingproperties. Because the reflective polarizer and absorbing polarizercombination effectively functions to select light having a certainpolarization state, it may be advantageous to have the reflection andabsorption axes, respectively, aligned (or in another sense, to havetheir transmission axes aligned).

Unfortunately, because conventional manufacturing processes result inrolls having transmission axes oriented orthogonally to one another,there are costly converting steps needed in order to provide a sheethaving an absorbing polarizer and a reflective polarizer with theirtransmission axes aligned. This is generally a sheet to roll laminationprocess, where the reflective polarizer is cut and rotated 90° in orderto align the axes. Not only is this process time-consuming and moreexpensive, but the converting steps also greatly increase the chance ofintroducing defects, which may lowers the yield or usable portion of thelaminated film stack.

FIG. 1 is an exploded top perspective view of a film stack including ahalf-wave retardation layer. Film stack 100 includes reflectivepolarizer 110, half-wave retardation layer 120, and absorbing polarizer130. The films of film stack 100 are in optical contact with oneanother: laminated, adhered, or otherwise disposed on each other. Insome embodiments, the films or layer of film stack 100 are attached withone or more of a pressure sensitive adhesive, an optically clearadhesive, a UV curable adhesive, or a polyvinyl alcohol type adhesive.In some embodiments the films or layers of film stack 100 aresubstantially the same dimensions, or coextensive. In some embodiments,the bottom surface of half-wave retardation layer 120 is disposed on thetop surface of reflective polarizer 110. And in some embodiments, thebottom surface of absorbing polarizer 130 is disposed on the top surfaceof half-wave retardation layer 120.

Reflective polarizer 110 may be any suitable reflective polarizer,including a wire-grid polarizer or a multilayer birefringent reflectivepolarizer. Suitable reflective polarizers include, for example, DualBrightness Enhancing Film (DBEF) or Advanced Polarizing Film (APF),available from 3M Company, St. Paul, Minn. Reflective polarizer 110 mayhave a transmission axis generally along the length direction of thefilm, as shown in FIG. 1. Reflective polarizer 110 may be chosen forother physical or optical characteristics, such as its reflection ortransmission spectra, structural durability, or delamination or warpresistance.

Half-wave retardation layer 120 may be any suitable construction.Generally, a half-wave retardation layer includes a layer of abirefringent material. In some embodiments, half-wave retardation layer120 is liquid crystal polymer coated on a film or substrate, such as acyclo-olefin polymer substrate, cellulose triacetate (triacetylcellulose), or polycarbonate. In some embodiments, as depicted in FIG.1, half-wave retardation layer 120 may have a slow axis orientedsubstantially 45° to the transmission axis of reflective polarizer 110.For the purposes of this application, an orientation of substantially45° and substantially 135° may be considered substantially equivalent asbetween two axes, given the bidirectional nature of a transmission axis.However, substantially 45° and substantially 135° may be used, whencomparing three or more axes, to distinguish two axes that may beoriented 90° to one another. Substantially 45° also may be understood tonot be limited to precisely 45°; instead, the alignment of the axes maybe within 10°, within 5°, or within 1° of 45°. Alignment may in somecases be a tradeoff between manufacturability (e.g., error tolerance)and optical performance, the appropriate balance being determineddepending on the desired application. Nonetheless, precise alignment maynot in fact be crucial in many applications, as shown in the modeledExamples. For purposes of this application, the terms retarder andretardation layer are used interchangeably.

In some cases half-wave retardation layer 120 may be an achromaticretardation layer. In other words, half-wave retardation layer 120 mayrotate or modulate polarization more or less depending on the wavelengthof incident light. In some embodiments, as depicted and described inconjunction with FIG. 3, the achromatic half-wave retardation layer mayinclude a two-part construction of two 90° -aligned three-quarters-waveand quarter-wave retardation layers.

Achromatic half-wave retardation layers may be used in some embodimentsto compensate for the non-linear wavelength-dependent modulation oflight in a conventional half-wave retardation layer, making thetransmission instead relatively flat, linear, or in conformance with orapproaching any desired spectrum. This may minimize or eliminate shiftsin color or other artifacts. In some embodiments the desiredachromaticity may be achieved through designing or selecting certainwavelength-specific retardance. For example, the achromatic half-waveretarder may have a retardance (i.e., difference in path length of oneof the orthogonal field components of incident light) of 200 nm for 400nm light and 400 nm for 800 nm light (corresponding to half awavelength). However, precise linear achromaticity is not necessary insome embodiments, and therefore the actual retardance values may bewithin 10%, within 7.5%, within 5%, or within 2% of the half wavelengthvalue. Similarly, for quarter-wave and three-quarter wave achromaticretarders, the actual retardance values of these retarders may be withinsimilar percentage ranges of their quarter's and three-quarters'wavelength value, respectively.

In some embodiments, reflective polarizer 110 may be turned, throughjudicious selection of materials and layer thickness of optical repeatunits, to compensate for the wavelength-dependent modulation of aconventional half-wave retardation layer. In other words, the tunedreflective polarizer may be tuned to reduce wavelength dispersion of thehalf-wave retardation layer. The optical thickness (physical thicknessmultiplied by the refractive index of a material) of each set ofmicrolayers, called an optical repeat unit, reflects light atwavelengths about twice its optical thickness through constructiveinterference. In designing a tuned reflective polarizer, the arrangementof these layers may be utilized to provide greater or lesser reflectionbased on wavelength.

Absorbing polarizer 130 may be any suitable material, including apolymeric material. In some embodiments, absorbing polarizer 130 mayinclude polyvinyl alcohol. In some embodiments, absorbing polarizer mayinclude polarizing elements, including polarizing or dichroic dyes. Thepolarizing elements may preferentially absorb light of a certainpolarization and preferentially transmit light of a second, orthogonalpolarization. The transmission axis, as depicted in FIG. 1, issubstantially orthogonal to the transmission axis of reflectivepolarizer 110 and substantially 45° (or 135°) from the slow axis ofhalf-wave retardation layer 120.

In some embodiments, additional or intermediate films, layers, orcomponents may be included; for example, diffusing layers, turninglayers, or substrate layers may be appropriate or desirable in someapplications. Optical film stack 100 may, in total, be any suitablethickness.

FIG. 2 is a front elevation view of the optical film stack of FIG. 1.Optical film stack 200, corresponding to optical film stack 100 of FIG.1 includes reflective polarizer 210, half-wave retardation layer 220,and absorbing polarizer 230. Light rays are shown to illustrate thebasic optical functionality and mechanism of the optical film stack.Reference to FIG. 1 may be useful to keep in mind the exemplaryorientation of the transmission and slow axes of the correspondinglayers contemplated in FIG. 2.

Unpolarized light 211 is incident on a first major surface of reflectivepolarizer 210. Unpolarized light need not have evenly or randomlydistributed polarization states; in fact, unpolarized light in someembodiments may actually be at least partially polarized light. However,for the ease of explanation in FIG. 2, and because unpolarized light 211in FIG. 2 has not yet passed through a polarizer that preferentiallytransmits a certain polarization state, unpolarized light 211 may betreated as light having an unknown or arbitrary polarization state ordistribution of polarization states.

Reflective polarizer 210, corresponding to reflective polarizer 110 inFIG. 1, preferentially transmits light having a polarization stateparallel to its transmission axis. Thus, first transmitted light 212having a first polarization state is preferentially transmitted. Lighthaving an orthogonal polarization state to the transmission axis ofreflective polarizer 210 is preferentially reflected as reflected light213. Note that in some embodiments the reflection or transmission ratiosmay vary based on wavelength or incidence angle, so in some embodimentsboth polarization states may be at least partially transmitted and atleast partially reflected. In some embodiments, this phenomenon may beexploited to control color or output angles.

First transmitted light 212 is incident on half-wave retardation layer220. Half-wave retardation layer 220 is configured to rotate or modulatethe polarization of first transmitted light 212 to orthogonallypolarized second transmitted light 214. For purposes of thisapplication, rotate and modulate are used to describe the overall effectof polarization state change; however, one skilled in the art willunderstand that the particular mechanisms, e.g., axis-specificretardation based on in-plane birefringence versus rotation due tohelical liquid crystal structure may be interchanged or combined in someembodiments without changing terminology. As described in conjunctionwith FIG. 1, in some embodiments this polarization rotation may bewavelength dependent, so achromatic half-wave retardation layers—thatis, half-wave retardation layers that alter this wavelength-dependentrotation—may be utilized.

Overall, second transmitted light 214 is transmitted through half-waveretardation layer 220, now having an orthogonally oriented polarizationstate to first transmitted light 212. In some embodiments, secondtransmitted light 214 may desirably be of similar intensity as firsttransmitted light 212, requiring low absorptive or reflective lossesfrom half-wave retardation layer 220. Second transmitted light 214 isthereafter incident on absorbing polarizer 230, which, referring againto FIG. 1, has a transmission axis oriented orthogonally to reflectivepolarizer 210. Because of the polarization rotation from half-waveretardation layer 220, however, the polarization state of lighttransmitted through reflective polarizer 210 is similarly orientedorthogonally to light incident on (and ultimately transmitted through)absorbing polarizer 230.

Absorbing polarizer 230 need not in fact be an absorbing polarizer;however, in some embodiments it may be desirable to minimize potentiallydistracting reflection in certain backlight configurations byconfiguring it as an absorbing polarizer. In some embodiments, thehalf-wave retardation layer 220 may transmit an appropriate distributionof polarization states as a function of wavelength for secondtransmitted light 214 so that output light 216 has a desired brightnessor color performance. Nonetheless, output light 216 is transmittedthrough absorbing 216 and in some embodiments may be further incident onother films or backlight components, including a pixilated liquidcrystal display. Essentially, together, reflective polarizer 210 andhalf-wave retardation layer 220 function to simulate a transmission axisoriented orthogonally to the actual transmission axis of reflectivepolarizer 210. Viewed differently, absorbing polarizer 230 and half-waveretardation layer 220 function together to simulate a transmission axisoriented orthogonally to the actual transmission axis of absorbingpolarizer 230.

FIG. 3 is an exploded top perspective view of another optical film stackincluding a retardation layer. In a sense FIG. 3 shows optical filmstack 300 including reflective polarizer 310, retarder film layer 320,liquid crystal layer 322 and absorbing polarizer 330. The function andconfiguration of optical film stack 300 is similar to optical film stack100 in FIG. 1; however, retarder film layer 320 and liquid crystal layer322 are used instead of half-wave retardation layer 120. This split intotwo layers in the view of FIG. 3 is for ease of explanation and theconfiguration described with respect to this figure may apply equallywhere a single-layer half-wave retardation layer is referenced. In otherwords, the two-layer construction described in FIG. 3 may befunctionally equivalent to a single-layer configuration and may besubstituted as appropriate. The bottom surface of retarder film layer320 may be disposed on the top surface of reflective polarizer 310, thebottom surface of liquid crystal layer 322 may be disposed on the topsurface of retarder film layer 320, and the bottom surface of absorbingpolarizer 330 may be disposed on the top of liquid crystal layer 322. Aswith other embodiments in this disclosure, suitable adhesives, includingoptically clear and pressure sensitive adhesives, laminations, or otherattachment mechanisms may be used, the layers still being considereddisposed on one another.

Retarder film layer 320 may in some embodiments be a three-quarter waveretardation layer. In some embodiments, retarder film layer 320 may be acyclo-olefin polymer retardation layer. The slow axis of retarder filmlayer 320 may be oriented as depicted in FIG. 3; that is, substantially45° from the transmission axis of reflective polarizer 310.

Liquid crystal layer 322 is disposed on retarder film layer 320 and mayfunction as a quarter wave retardation layer, with its slow axisoriented orthogonally to the slow axis of retarder film layer 320. Insome embodiments, liquid crystal layer 322 may be essentially a liquidcrystal coating on retarder film layer 320. In some embodiments, liquidcrystal layer 322 may include reactive mesogen liquid crystal. In someembodiments, the alignment and thickness of liquid crystal layer 322 maybe designed or selected in order to simulate a half-wave retardationlayer in conjunction with retarder film layer 320. In some embodiments,these characteristics of liquid crystal layer may be selected toprovide, in conjunction with retarder film layer 320, an achromatichalf-wave retardation layer as described elsewhere. Intricate control ofthe overall properties of the optical film stack may be possible throughthe material selection, alignment, and thickness of the liquid crystallayer 322. Therefore, in some embodiments, liquid crystal layer 322 maybe designed or adjusted to provide desired color performance orbrightness.

EXAMPLES Example 1

Simulations were performed to determine transmittance as a function ofwavelength for film stacks used in LCD displays. The simulations werecarried out using TechWiz LCD 1D Plus, which is simulation softwarecommercially available from Sanayi System Co. Ltd. (Incheon, Korea). TheLambertian light source option provided by the TechWiz Database was usedin all simulations.

The film stacks included a retarder between an absorbing polarizer and areflective polarizer where the polarizers were disposed such that thepass axis of the reflective polarizer was orthogonal to the pass axis ofthe absorbing polarizer. The retarder was modeled as a ¾ wavelengthcyclo-olefin polymer (COP) retarder with a liquid crystal (LC) polymerretarder deposited on the COP retarder such that the slow axis of the LCretarder was at 90 degrees relative to the slow axis of the COPretarder. The ¾ wavelength retarder was disposed so that its slow axiswas at 45 degrees relative to the pass axis of the reflective polarizerand the LC retarder was disposed so that its slow axis was at −45degrees relative to the pass axis of the reflective polarizer.

The film stacks modeled in the simulation used a COP retarder with theindices of refraction given in Table 1, which were chosen asrepresentative of a COP retarder available commercially from Zeon Corp.The x-axis refers to the slow axis, the y-axis is perpendicular to theslow axis and in the plane of the retarder film, and the z-axis is inthe thickness direction.

TABLE 1 Wavelength (nm) n_(x) n_(y) n_(z) 450 1.52552 1.51847 1.51621550 1.52539 1.51842 1.51619 650 1.52534 1.51839 1.51618

To simulate a ¾ wavelength retarder, the thickness, d, of the COPretarder was taken to be 70 microns which gave the retardation valuesshown in Table 2, where Re=(n_(x)−n_(y)) d andRth=[(n_(x)+n_(y))/2−n_(z)]d.

TABLE 2 Wavelength (nm) Re (nm) Rth (nm) 450 493.5 404.9 550 487.9 400.1650 486.5 397.9

The LC used in the LC retarder was taken to be 5CB(4-pentyl-4′-cyanobiphenyl) having the extraordinary (n_(e)) andordinary (n_(o)) indices of refraction given in Table 3.

TABLE 3 Wavelength (nm) n_(e) n_(o) 450 1.76933 1.55909 550 1.723801.53983 650 1.70342 1.53069

The thickness of the LC layer was varied from 1.04 microns to 1.28microns in the simulations. The retardation at 1.04 microns is given inTable 4 and the retardation at 1.28 microns is given in Table 5.

TABLE 4 Wavelength (nm) Re (nm) Rth (nm) 450 218.6 109.3 550 191.3 95.7650 179.6 89.8

TABLE 5 Wavelength (nm) Re (nm) Rth (nm) 450 269.1 134.6 550 235.5 117.7650 221.1 110.5

For comparison, a COP retarder film with the indices of refraction givenin Table 1 with a thickness of 39 microns was modeled. This provided anapproximately ½ wave retarder with the retardation values given in Table6.

TABLE 6 Wavelength (nm) Re (nm) Rth (nm) 450 275.0 225.6 550 271.8 222.9650 271.1 221.7

The conventional structure of a reflective polarizer and an absorbingpolarizer without a retarder between the two polarizers and with thepass axis of the reflective polarizer and the absorbing polarizeraligned was also simulated.

The transmittance as a function of wavelength for various thicknesses ofthe LC layer is provided in Table 7 and in FIG. 4. It can be seen thatcompared to the case of no retarder, the ½ wave COP retarder produced astrong wavelength dependence. The use of achromatic retarderssignificantly improved the results.

TABLE 7 LC Thickness Wavelength (nm) (microns) 400 450 500 550 600 650700 1.04 0.276 0.304 0.334 0.357 0.370 0.374 0.370 1.06 0.285 0.3100.338 0.359 0.370 0.372 0.368 1.08 0.293 0.316 0.341 0.360 0.370 0.3710.366 1.1 0.300 0.321 0.344 0.362 0.370 0.369 0.363 1.12 0.307 0.3260.347 0.362 0.369 0.368 0.361 1.14 0.313 0.330 0.349 0.363 0.368 0.3660.358 1.16 0.318 0.334 0.351 0.363 0.367 0.363 0.355 1.18 0.323 0.3370.352 0.363 0.365 0.361 0.352 1.2 0.326 0.339 0.353 0.362 0.364 0.3580.349 1.22 0.328 0.341 0.354 0.361 0.362 0.356 0.345 1.24 0.330 0.3430.354 0.360 0.360 0.353 0.342 1.26 0.331 0.344 0.354 0.359 0.357 0.3500.338 1.28 0.331 0.344 0.353 0.357 0.354 0.346 0.335 No retarder 0.3340.346 0.356 0.365 0.372 0.378 0.383 ½ wave 0.221 0.306 0.349 0.364 0.3630.353 0.337 COP, no LC

Color was characterized in terms of CIE Chromaticity Yxy coordinates.The color shift parameters Δx and Δy relative to the case with alignedreflective and absorbing polarizer without a retarder is given in Table8 and in FIG. 5 for on-axis viewing (angular coordinates: Θ=0°, Φ=0°),horizontal off-axis viewing (angular coordinates: Θ=45°, Φ=0°), anddiagonal off-axis viewing (angular coordinates: Θ=45°, Φ=45°). The caseof aligned polarizers with no retarder has a color shift of zero bydefinition, while the case of a ½ wave COP retarder between rotatedpolarizers has off axis color shifts that are off the scale of FIG. 5.

TABLE 8 Θ = 45°, Φ = 0° Θ = 45°, Φ = 45° LC Θ = 0°, Φ = 0° (Horizontal(Diagonal Thickness (On-axis) off-axis) off-axis) (microns) Δx Δy Δx ΔyΔx Δy 1.04 0.0094 0.012 0.0137 0.0144 −0.0118 −0.0061 1.06 0.0077 0.01030.0119 0.0126 −0.0113 −0.0053 1.08 0.0062 0.0088 0.0103 0.0109 −0.0106−0.0044 1.1 0.0047 0.0073 0.0087 0.0094 −0.0099 −0.0033 1.12 0.00340.0059 0.0072 0.0079 −0.0091 −0.002 1.14 0.0021 0.0047 0.0058 0.0066−0.0081 −0.0006 1.16 0.0009 0.0035 0.0046 0.0053 −0.007 0.0009 1.18−0.0002 0.0024 0.0034 0.0041 −0.0057 0.0027 1.2 −0.0012 0.0014 0.00230.0031 −0.0043 0.0046 1.22 −0.0021 0.0005 0.0013 0.0021 −0.0027 0.00671.24 −0.003 −0.0003 0.0003 0.0012 −0.001 0.009 1.26 −0.0038 −0.001−0.0005 0.0004 0.001 0.0116 1.28 −0.0045 −0.0016 −0.0013 −0.0003 0.00310.0143 ½ wave 0.0054 0.0144 0.0081 0.0173 −0.017 −0.011 COP, no LC

Example 2

Simulations were performed as in Example 1, except that the thickness ofthe LC retarder was fixed at 1.22 microns and the angle between the slowaxis of the COP retarder and the pass axis of the reflective polarizer(hereinafter, the COP angle) was varied from 45 to 60 degrees. The anglebetween the slow axis of the LC retarder and the pass axis of thereflective polarizer (hereinafter, the LC angle) was varied from −45 to−30 degrees keeping the relative angle between the slow axis of the COPretarder and the slow axis of the LC retarder at 90 degrees (i.e., theLC angle was the COP angle minus 90 degrees).

Transmittance versus wavelength for several different COP angles isshown in FIG. 6. The relative transmittance (relative to the COP angleof 45 degrees) is given in Table 9. From FIG. 6 and Table 9, it can beseen that the relative transmittances initially drop very slowly as theCOP angle moves away from 45 degrees.

TABLE 9 COP Angle (deg.) 45 46 47 48 50 55 60 Relative 100 100 100 99 9788 75 Transmittance (%)

The color shift relative to the case where the COP angle is 45 degreesis given in Table 10 and FIG. 7 where it can be seen that the colorshift is not very sensitive to COP angle for angles between 45 degreesand about 50 degrees.

TABLE 10 Θ = 45°, Φ = 0° Θ = 45°, Φ = 45° Θ = 0°, Φ = 0° (Horizontal(Diagonal COP Angle (On-axis) off-axis) off-axis) (deg.) Δx Δy Δx Δy ΔxΔy 45 0 0 0 0 0 0 46 0.0000 0.0000 −0.0007 −0.0011 0.0002 0.0003 470.0000 0.0000 −0.0014 −0.0022 0.0006 0.0007 48 0.0000 0.0000 −0.0023−0.0035 0.0010 0.0012 50 0.0000 0.0000 −0.0043 −0.0065 0.0022 0.0025 550.0000 0.0000 −0.0107 −0.0155 0.0064 0.0072 60 0.0000 0.0000 −0.0189−0.0265 0.0116 0.0129

Example 3

Simulations were performed as in Example 1, except that the thickness ofthe LC retarder was fixed at 1.22 microns and the angle between the slowaxis of the COP retarder and the pass axis of the reflective polarizerwas varied from 45 to 35 degrees, while the angle between the slow axisof the LC retarder and the pass axis of the reflective polarizer wasvaried from −45 to −35 degrees. The relative angle between the slow axisof the COP retarder and the slow axis of the LC retarder varied from 90degrees to 70 degrees.

Transmittance versus wavelength for three combinations of COP angle andLC angle are shown in FIG. 5. The color shift relative to the case wherethe COP angle was 45 degrees and the LC angle was −45 degrees is givenin Table 11. From FIG. 8 and Table 11, it can be seen that thetransmittance and color shift are not very sensitive to small deviationsof the COP and LC angles from 45 degrees and −45 degrees, respectively.

TABLE 11 COP Θ = 45°, Φ = 0° Θ = 45°, Φ = 45° An- LC Θ = 0°, Φ = 0°(Horizontal (Diagonal gle Angle (On-axis) off-axis) off-axis) (deg.)(deg.) Δx Δy Δx Δy Δx Δy 45 −45 0 0 0 0 0 0 40 −40 −0.0021 −0.00020.0037 0.0025 −0.0124 −0.0151 35 −35 −0.0077 −0.0007 0.0011 0.0013−0.0282 −0.0338

The following are exemplary embodiments according to the presentdisclosure:

-   Item 1. An optical film stack, comprising:

a reflective polarizer having a top surface, a bottom surface, atransmission axis, and a reflection axis;

an achromatic half-wave retardation layer having a top surface, a bottomsurface disposed on the top surface of the reflective polarizer, and aslow axis oriented substantially 45° with respect to the transmissionaxis of the reflective polarizer; and

an absorbing polarizer having a bottom surface disposed on the topsurface of the achromatic half-wave retardation layer and a transmissionaxis oriented substantially 90° with respect to the transmission axis ofthe reflective polarizer.

-   Item 2. An optical film stack, comprising:

a reflective polarizer having a top surface, a bottom surface, atransmission axis, and a reflection axis;

a first retardation layer having a top surface, a bottom surfacedisposed on the top surface of the reflective polarizer, and a slow axisoriented substantially 45° with respect to the transmission axis of thereflective polarizer;

a second retardation layer having a top surface, a bottom surfacedisposed on the top surface of the first retardation layer, and a slowaxis oriented substantially 45° with respect to the transmission axis ofthe reflective polarizer and oriented substantially 90° with respect tothe slow axis of the first retardation layer; and

an absorbing polarizer having a bottom surface disposed on the topsurface of the second retardation layer and a transmission axis orientedsubstantially 90° with respect to the transmission axis of thereflective polarizer;

wherein together the first and second retardation layers are configuredas an achromatic half-wave retardation layer.

-   Item 3. An optical film stack, comprising:

a tuned reflective polarizer having a top surface, a bottom surface, atransmission axis, and a reflection axis;

a half-wave retardation layer having a top surface, a bottom surfacedisposed on the top surface of the reflective polarizer, and a slow axisoriented substantially 45° with respect to the transmission axis of thereflective polarizer; and

an absorbing polarizer having a bottom surface disposed on the topsurface of the achromatic half-wave retardation layer and a transmissionaxis oriented substantially 90° with respect to the transmission axis ofthe reflective polarizer;

wherein the tuned reflective polarizer is tuned to reduce wavelengthdispersion of the half-wave retardation layer.

-   Item 4. The optical film stack of item 2, wherein the first    retardation layer is a three-quarters-wave retardation layer and the    second retardation layer is a quarter-wave retardation layer.-   Item 5. The optical film stack as in any of items 1-3, further    comprising a quarter-wave retardation layer disposed on the bottom    surface of the reflective polarizer.-   Item 6. The optical film stack as in either of items 2 or 4, wherein    the second retardation layer includes a liquid crystal layer.-   Item 7. The optical film stack as in any of items 2, 4, or 6,    wherein the first retardation layer includes a film.-   Item 8. The optical film stack of item 6, wherein the liquid crystal    layer has a thickness of between 1 and 1.3 microns.-   Item 9. The optical film stack of item 6, wherein the liquid crystal    layer has a thickness of between 1.18 and 1.24 microns.-   Item 10. The optical film stack of item 1, wherein a retardation    value of the achromatic half-wave retardation layer varies directly    with a wavelength of visible light.-   Item 11. The optical film stack of item 1, wherein the achromatic    half-wave retardation layer is attached to the absorbing polarizer    with at least one of a pressure sensitive adhesive, a UV curable    adhesive, or a polyvinyl alcohol type adhesive.-   Item 12. The optical film as in any of items 1-3, wherein    substantially 45° means not less than 35° and not more than 55°.-   Item 13. The optical film as in any of items 1-3, wherein    substantially 45° means not less than 40° and not more than 50°.-   Item 14. The optical film as in any of items 1-3, wherein    substantially 45° means not less than 44° and not more than 46°.-   Item 15. The optical film as in any of items 1-3, wherein    substantially 90° means not less than 80° and not more than 100°.-   Item 16. The optical film as in any of items 1-3, wherein    substantially 90° means not less than 85° and not more than 95°.-   Item 17. The optical film as in any of items 1-3, wherein    substantially 90° means not less than 89° and not more than 91°.-   Item 18. A roll of film, comprising the optical film stack as in any    one of the preceding items.

Descriptions for elements in figures should be understood to applyequally to corresponding elements in other figures, unless indicatedotherwise. The present invention should not be considered limited to theparticular embodiments described above, as such embodiments aredescribed in detail in order to facilitate explanation of variousaspects of the invention. Rather, the present invention should beunderstood to cover all aspects of the invention, including variousmodifications, equivalent processes, and alternative devices fallingwithin the scope of the invention as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. An optical film stack, comprising: a reflectivepolarizer having a top surface, a bottom surface, a transmission axis,and a reflection axis; an achromatic half-wave retardation layer havinga top surface, a bottom surface disposed on the top surface of thereflective polarizer, and a slow axis oriented substantially 45° withrespect to the transmission axis of the reflective polarizer; and anabsorbing polarizer having a bottom surface disposed on the top surfaceof the achromatic half-wave retardation layer and a transmission axisoriented substantially 90° with respect to the transmission axis of thereflective polarizer.
 2. The optical film stack of claim 1, furthercomprising a quarter-wave retardation layer disposed on the bottomsurface of the reflective polarizer.
 3. The optical film stack of claim1, wherein a retardation value of the achromatic half-wave retardationlayer varies directly with a wavelength of visible light.
 4. A roll offilm, comprising the optical film stack of claim
 1. 5. An optical filmstack, comprising: a reflective polarizer having a top surface, a bottomsurface, a transmission axis, and a reflection axis; a first retardationlayer having a top surface, a bottom surface disposed on the top surfaceof the reflective polarizer, and a slow axis oriented substantially 45°with respect to the transmission axis of the reflective polarizer; asecond retardation layer having a top surface, a bottom surface disposedon the top surface of the first retardation layer, and a slow axisoriented substantially 45° with respect to the transmission axis of thereflective polarizer and oriented substantially 90° with respect to theslow axis of the first retardation layer; and an absorbing polarizerhaving a bottom surface disposed on the top surface of the secondretardation layer and a transmission axis oriented substantially 90°with respect to the transmission axis of the reflective polarizer;wherein together the first and second retardation layers are configuredas an achromatic half-wave retardation layer.
 6. The optical film stackof claim 5, wherein the first retardation layer is a three-quarters-waveretardation layer and the second retardation layer is a quarter-waveretardation layer.
 7. The optical film stack as in claim 5, wherein thesecond retardation layer includes a liquid crystal layer.
 8. The opticalfilm stack of claim 7, wherein the liquid crystal layer has a thicknessof between 1 and 1.3 microns.
 9. The optical film stack as in claim 5,wherein the first retardation layer includes a film.